I semi-agree about sandbox games- I think the real problem is they're much harder to do well than a VN, and when they're done badly they're a miserable experience, and a lot of people have been burned once or twice and swear off the whole genre.
But on topic, I think jealousy between love interests. A lot of games give the player character a harem and either ignore the issue of how all these girls feel about him simultaneously dating them or just make them all magically okay with it. I kinda like it when there's some real drama around how different characters react to knowing they're not your one and only.
Also incest- I like it, but part of the appeal is the taboo and seeing how the characters overcome it, and a lot of games either pretend it isn't there or give lip service to it ("she's my sister, this is so wrong!" continues thrusting). Which takes some of the punch out for me; I'd like more games where the characters actually struggled with it a bit.